There is a considerable requirement for bi-metallic tubing, specifically for use in drill pipe and distribution pipeline in the oil and gas industries. Conventionally, such tubes consist of a principal metallic tube made from a first metal to which is bonded a metallic layer made from a second metal, the second metal being a more expensive corrosion-resistant material. The use of such tubing leads to reduction of costs by removing the need to manufacture corrosion resistant tubing which would be more expensive were it to consist entirely of the more expensive material and be made in a wall thickness which is sufficient to meet normal pressure requirements. The corrosion resistant layer will be on the inner surface of the tube through which the corrosive material passes.
Bi-metallic tubing is produced commercially by several methods including co-extrusion, centrifugal casting, co-drawing and internal weld overlaying, but co-extrusion is the predominant method of manufacture. The co-extrusion method consists of placing a first tube of corrosion resistant metal into the bore of a second and much thicker walled tube of less expensive metal, usually steel. The external diameter of the outer tube will be a close fit into the container of an extrusion press and will typically be some 300 mm to 400 mm. The internal bore will vary and be dependent upon the press mandrel being used to produce the specific bore for the extruded `shell` to be formed. The length of the loose composite, i.e. first and second tubes, will be that required for the container of the extrusion press, and the proportional wall thickness of the two metallic tubes will be identical to that desired in the final tube to be produced. The interface of the tubes is sealed prior to heating the composite which is then extruded. On extrusion, the two metals become bonded at the interface.
Similarly, bonding of a loose composite of an inner and outer tube of dissimilar but compatible metals by heating and rolling the tubes over a mandrel to press the inner tube against the outer has been described in U.S. Pat. No. 4,162,758.
The principal limitation of these processes is that, to be successful, the two metals being used must be compatible for solid state bonding by these processes. For compatibility only small differences in the mechanical properties and atomic spacing of the two metals is tolerable. The choice of metals which can be used is therefore restricted.
An alternative method of producing the bi-metallic tubing is explosive bonding which is much less restricted and, because of the higher pressures involved, allows metal combinations with much more widely divergent mechanical properties and atomic spacing to be bonded. This method is implemented, for example, by placing a corrosion resistant tube within a steel tube and centralising the two tubes. The outside and inside diameters of the inner and outer tubes respectively are dimensioned such that, on centralising the two tubes, an annular gap exists between the tubes. In some cases, for metallurgical reasons such as, for example, migration of carbon from carbon steel to a corrosion resistant metal, a third tube of a different metal may be interposed between the inner and outer tubes with an annular gap between adjacent tubes. From this point the method continues via one of two methods, i.e. expansion or implosion.
The expansion method is described in UK Patent Nos. 2,209.978 and 2,209,979 and the implosion method is described in "A Fabrication Process for the Production of Zirconium Bimetal Tube for Cl.sub.2 and H.sub.2 S Gas Wells", by R Hardwick and C T Wang, in the Proceedings of The High Energy Rate Forming Conference, 1984, pp 189-194.
When bonding the expansion method an explosive charge is disposed within at least a portion of the bore of an inner tubular metal component to be expanded, and exploded to expand the inner component radially to collide and bond with the interior of the outer component.
In the implosion method an external annular explosive charge is disposed around the portion of tube to be imploded and fired. There are, however, limitations to both these methods of explosive bonding.
In the expansion method, for example, the inner tube bore defines the maximum volume of explosive which can be contained within it. If the wall thickness of the inner tube is sufficiently thick, a situation will arise where the tube bore cannot contain sufficient explosive to achieve bonding. This therefore defines a relationship between the tube bore, the wall thickness, and the material used.
Further, the outer tube wall thickness should be sufficiently thick if the outer tube itself is not to be expanded by the explosive charge. Should expansion of the outer tube occur, not only is dimensional control lost but the collision pressure occurring at the interface between the tubes is reduced, so leading to reduced bond quality. This problem may be overcome by use of an external die, as suggested in GB A 2,209,979. However this solution is time consuming, labour intensive and expensive.
A further disadvantage of the expansion method is that the detonation rate of the explosive is accelerated by the progressive increase in pressure within the tube bore. A situation may therefore occur where the detonation rate increases to a point beyond the upper limit for bonding. Consequently the length of tube together with the relative thicknesses of the inner and outer tubes which may be bonded by the expansion method is limited and is generally too short for practical requirements.
The method of implosion also suffers from a number of disadvantages. For example, the wall thickness of the outer tube being imploded is limited because of the requirement that, in order to ensure bonding, the interfacial annular gap should be a minimum of around 20% of the outer tube wall thickness. Thus if the outer wall is thick, the gap will be substantial and a situation will arise where the degree of contraction required of the outer tube is excessive. Surface wrinkling may, therefore, occur during contraction to depths which will not be removed by the bonding process, the avoidance of such wrinkling being essential to the explosive bonding process.
A further disadvantage of the implosion method previously used is that the upper limit of tube length which can practically be achieved is 3-4 meters. This is due to the difficulty in attaining a uniform explosive density along the length of the annular charge. Variations in explosive density may affect the detonation velocity and so cause the detonation front passing down the annular gap to be destabilised, and increasingly distorted as a function of distance. This continues until the associated collision front at the interface below the detonation front is no longer travelling exclusively in a longitudinal direction but also circumferentially in opposing directions. When these opposing fronts meet at a diametrically opposite point, adiabatic compression of air in front of the collision front causes excessive melting of the surface, preventing metal-to-metal bonding and also causing potential rupture of the inner tube. Consequently the implosion method limits the thickness of the outer tube and the length of the bi-metallic tube which can be produced.
Both the expansion and implosion methods are relatively expensive as they are extremely labour intensive. Further, the length limitations mean tube lengths are short, resulting in a high frequency of expensive prefabricated joints in the extended pipeline lengths which are usually required to be supplied for on-site installation. These methods have, therefore, hitherto found limited practical application and have only been used where technical quality requirements were of paramount importance.
In our co-pending United Kingdom patent application No. 9105651-5 a method for producing bi-metallic tubing which overcomes the abovementioned limitations is described. In this method the interior cylindrical surface of a tube of a first metal is explosively bonded to the exterior cylindrical surface of a substantially incompressible billet of a second metal by implosion of the tube of the first metal onto the substantially incompressible billet to form a cylindrical bonded composite. The composite is sub-divided and each of the several lengths of the cylindrical bonded composite is subsequently hot-extruded at an elevated temperature to form an extruded bi-metal shell of extended length. This shell is sub-divided into lengths, each of which is subsequently placed within the bore of a hollow billet of metal compatible with the first metal, so as to form an annular interface between exterior cylindrical surface of the shell and the interior cylindrical surface of the hollow billet. The annular interface is subsequently sealed so as to form an assembled composite, the composite then being co-extruded at an elevated temperature to form the required bi-metal tube. This method removes the limitations on the length of tube which can be produced and renders the whole production process commercially viable, the original high cost per unit length of the bonding process now having been reduced to a relatively small cost over the several extended lengths of the total final product.
The maximum diameter of tube which can be produced by this extrusion route is, however, limited by the diameter of the container of the available extrusion press and, in practice, it is unusual for tube diameters greater than 300 mm to be extruded.